Pre-Press color proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high volume printing press. These representative images may require correction and may be reproduced several times to satisfy customer requirements.
One such commercially available image processing apparatus, shown in commonly assigned U.S. Pat. No. 5,268,708, has half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media in which dye from a sheet of dye donor material is transferred to thermal print media by applying thermal energy to the dye donor material. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media, in roll form, from the material assembly or carousel. The thermal print media is measured and cut into sheets of a required length, transported to the vacuum imaging drum, registered, and wrapped around and secured onto the vacuum imaging drum. Next a length of dye donor material, in roll form, is metered out of the material supply assembly, measured, and cut into sheets of a required length. The dye donor material is transported to and wrapped around the vacuum imaging drum, such that it is superposed in registration with the thermal print media.
The thermal print media and the dye donor material are retained on the spinning vacuum imaging drum while it is rotated past the scanning subsystem to expose the thermal print media. The translation drive traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the image on the thermal print media.
After the image has been written on the thermal print media, the dye donor material is removed from the vacuum imaging drum. This is done without disturbing the thermal print media beneath it. The dye donor material is then transported out of the image processing apparatus by the dye donor material exit transport. Additional sheets of dye donor material, each a different color, are sequentially superimposed with the thermal print media on the vacuum imaging drum and imaged onto the thermal print media as previously described, until the intended image is completed. The completed image on the thermal print media is unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the exit transport.
The vacuum imaging drum is cylindrical in shape and includes a hollowed-out interior portion, and a plurality of holes extending through a surface of the drum for applying a vacuum from the interior of the vacuum imaging drum, for supporting and maintaining the position of the thermal print media and dye donor sheet material as the vacuum imaging drum rotates. The ends of the vacuum imaging drum are enclosed by cylindrical plates. The cylindrical end plates are each provided with a centrally disposed spindle which extends outwardly through support bearings and are supported by the lathe bed scanning frame. The drive end spindle extends through the support bearing and is stepped down to receive a DC drive motor armature which is held on by means of a nut. A DC motor stator is held stationary by the lathe bed scanning frame, encircling the armature to form a reversible, variable speed DC drive motor for the vacuum imaging drum. At the end of the spindle an encoder is mounted to provide timing signals to the image processing apparatus. The opposite spindle is provided with a central vacuum opening, which is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame. The vacuum fitting has an extension which extends within but is closely spaced from the vacuum spindle, thus forming a small clearance. With this configuration, a slight vacuum leak is provided between the outer diameter of the vacuum fitting and the inner diameter of the opening of the vacuum spindle. This assures that no contact exists between the vacuum fitting and the vacuum imaging drum which might impart uneven movement or jitters to the vacuum imaging drum during its rotation.
The opposite end of the vacuum fitting is connected to a high-volume vacuum blower which is capable of producing 50-60 inches of water at an air flow volume of 60-70 cfm. The vacuum required varies during loading, scanning, and unloading of the thermal print media and the dye donor materials. With no media loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 10-15 inches of water. With just the thermal print media loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 20-25 inches of water. This level is required such that when a dye donor sheet material is removed, the thermal print media does not move, otherwise color to color registration will not be maintained. With both the thermal print media and dye donor sheet material completely loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 50-60 inches of water.
The outer surface of the vacuum imaging drum is provided with an axially extending flat, which extends approximately 8 degrees around the vacuum imaging drum circumference. The vacuum imaging drum is also provided with a circumferential recess which extends circumferentially from one side of the axially extending flat circumferentially around the vacuum imaging drum to the other side of the axially extending flat, and from approximately one inch from one edge of the vacuum imaging drum to approximately one inch from the other edge of the vacuum imaging drum. The thermal print media when mounted on the vacuum imaging drum is seated in the circumferential recess and therefor the circumferential recess has a depth substantially equal to the thermal print media thickness seated there within which is approximately 0.004 inches in thickness.
The purpose of the circumferential recess on the vacuum imaging drum surface is to eliminate any creases in the dye donor material, as it is drawn down over the thermal print media during the loading. This assures that no folds or creases will be generated in the dye donor materials which could extend into the image area and seriously adversely affect the image. The circumferential recess also substantially eliminates the entrapment of air along the edge of the thermal print media, where it is difficult for the vacuum holes in the vacuum imaging drum surface to assure the removal of the entrapped air. Any residual air between the thermal print media and the dye donor material, can also adversely affect the intended image.
The purpose of the vacuum imaging drum axially extending flat is two fold, it assures that the leading and trailing ends of the dye donor material are protected from the effects of air drag during high speed rotation of the vacuum imaging drum during the imaging process. Without the axially extending flat, the air drag will tend to lift the leading or trailing edges of the dye donor material. The vacuum imaging drum axially extending flat also ensures that the leading and trailing ends of the dye donor sheet material are recessed from the vacuum imaging drum periphery. This reduces the chance of the dye donor material contacting other parts of the image processing apparatus, such as the printhead, causing a jam, possible loss of the intended image, or catastrophic damage to the image processing apparatus.
The vacuum imaging drum axially extending flat also acts to impart a bending force to the ends of the dye donor materials when they are held onto the vacuum imaging drum surface by vacuum from within the interior of the vacuum imaging drum. Consequently when the vacuum is turned off to that portion of the vacuum imaging drum, the end of the dye donor material will tend to lift from the surface of the vacuum imaging drum.
Although the present image processing apparatus is satisfactory, it is not without drawbacks. Throughput, the number of intended images per hour, is limited by the vacuum imaging drum rotational speed. With some constraints imposed by the technology itself, the faster the vacuum imaging drum rotates without the centrifugal forces or increased air turbulence separating the thermal print media and the dye donor sheet material from the vacuum imaging drum, the faster the intended image can be exposed onto the thermal print media, thus increasing the throughput of the image processing apparatus. However, with the existing image processing apparatus, the physical characteristics of the thermal print media, the interface of the axially extending flat, circumferential recess, and the dye donor material, limit the rotational speeds that are possible. At high rotational speeds, in excess of 1000 RPM, the centrifugal forces and air turbulence lift or separate the dye donor materials from the vacuum imaging drum surface at the corners of the sheet of dye donor material.
One approach to solving the above problem is adding external clamping components to hold the thermal print media and the dye donor sheet material to the vacuum imaging drum. This, however, adds increased cost and introduces mechanical complexity to the vacuum imaging drum design. This solution may also cause the vacuum imaging drum to go out of round as much as 80 microns, which would not allow the image processing apparatus to meet quality specifications. (The image processing apparatus tolerance requirement for focus is approximately 10 microns.) Clamping the thermal print media and the dye donor material would also introduce a clearance problem since the working distance of the printhead to the surface of the thermal print media loaded on the vacuum imaging drum is approximately 0.030 inches.
Another way to prevent the increased air turbulence and centrifugal force from separating the dye donor sheet material from the rotating vacuum imaging drum would be to add more vacuum holes or enlarge the diameter of the vacuum holes. This, however, would require an increase in the vacuum level in the interior of the vacuum imaging drum. A higher vacuum will increase in the cost of the blower that produces the vacuum, require complex vacuum coupling, adding additional cost, and higher customer operating costs with increased electrical consumption. In addition, there is a limit to how high the vacuum level can be without distorting the media, hence decreasing the image quality of the intended image.
Another problem with present vacuum imaging drum designs is that the corners of the dye donor material may cause a flute or ridge-like ripple to be formed in the dye donor material as the leading and trailing edge is drawn down at the axially extending flat. This fluting, where it extends over the thermal print media, will adversely affect the quality of the image.